Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

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Vertical reference line

Movable line MRP

FIGURE 4-15 Centration devices using a shadow projection of the pattern work best when the pattern size equals the frame eyesize. When a pattern is smaller than the frame’s eyesize, compensation must be made by visualizing whether the lens will cut out. In this figure, the dotted line represents the “visualized” increase in pattern size that must be estimated to determine whether the lens blank size is sufficient. As shown, the lens blank will be too small. MRP, Major reference point.

Boxing

center

C H A P T E R 4 C E N T R AT I O N O F S I N G L E V I S I O N L E N S E S

decentration inward occurs, the minimum-sized lens must be 1 mm larger temporally. However, because uncut lens blanks are made round—with the optical center in the middle of the blank—they must increase equally in all directions. Therefore for every millimeter of decentration, the MBS also must increase 2 mm (Figure 4-17).

In addition, allowing an extra 2 mm is advisable for the possibility of lens chipping or other error. The extra 2 mm is only 1 mm on each side, so really only 1 mm of leeway exists.

Written mathematically and not including the extra 2 mm, the MBS is the following:

MBS = ED + 2(decentration)

Written to include the extra 2 millimeters, the formula is as follows:

MBS = ED + 2(decentration) + 2

Blank size charts and devices are available that allow the frame to be used in the determination of lens blank size. One such chart, and the explanation of how to use it, is shown in Figure 4-18.

LENSES OF INSUFFICIENT SIZE

Sometimes a lens blank is only slightly too small. A temptation exists to use the lens blank anyway by moving it

ED

OC

outward slightly. This causes the distance between optical centers to be larger than ordered and is called “pushing” the PD. By pushing the PD, the small lens blank may be used. But because the lens optical center will be decentered from its intended position before the eye, unwanted prism may result.

The amount of prism that will be induced by altering the ordered PD varies considerably with the power of the lens being used. The amount of prism that results is predictable using Prentice’s Rule.

INCORRECT INTERPUPILLARY DISTANCE

As mentioned in Chapter 2, Prentice’s Rule states the amount of prism ( ) induced by decentration of a lens. Prism amount is equal to the number of centimeters

(c) the lens is moved times the power of the lens (F). In equation form this appears as follows:

= cF

The stronger the power of the spectacle lens (F), the more unwanted prism will be induced for the same amount of lens decentration (c). Once this relationship is understood, it is relatively easy to estimate the amount of prism induced by a given amount of lens decentration.

Example 4-7

A –5.00 D lens blank is not large enough to cut out. If the PD is “pushed” 2 mm out for this lens, how much unwanted prism is induced?

Solution

The amount of unwanted prism can be calculated with the following equation:

= cF

The absolute value for F is used, so 5.00 is substituted for F in the equation. The decentration amount per lens is 2 mm. Because c is measured in centimeters and 2 mm = 0.2 cm, the equation becomes the following:

=(0.2) (5.00)

=1.00

A prism has both an amount and a base direction. For this eye the base direction will be base in. This is evident when the lens is seen in cross-section. This cross-section is shown schematically in Figure 4-19.

Base-in prism forces the eyes of the wearer to turn outward to keep from seeing double. For example, a pair of glasses with great amounts of unwanted prism is dispensed. If this happened, it would be better if the lenses were uncomfortable and rejected. The worst

result would be for the wearer to gradually become accustomed to the prism. This would contribute to either an outward deviation of the eyes or suppression of one eye with a resultant drop in its visual acuity. Although it is unlikely that the small prismatic amount in the previous example would cause such problems, the degree of difficulty encountered depends on the amount of prism induced and the visual condition of the wearer.

Example 4-8

A lens blank with a power of –0.25 D is too small and won’t cut out. What would the consequences be of “pushing” the PD by 2 mm?

Solution

Calculations are done in the same manner as before, but this time the power substituted in the equation is 0.25 instead of 5.00.

FIGURE 4-20 The value of a lens in a given meridian often can be better visualized through the use of a power cross system.

=cF

=(0.2) (0.25)

=0.05

cylinder axis in relation to the direction of decentration of the lens is important. In this case the lens is being decentered in the horizontal meridian. The practitioner needs to know the power of the lens in the horizontal meridian to calculate prism amount.8

Because the cylinder power is at right angles to the axis of the cylinder, the power (F) used in Prentice’s Rule is +4.00 D (Figure 4-20).

The prism induced by pushing the PD for this lens blank will be as follows:

=cF

=(0.2) (4)

=0.80

The amount of prism induced is 0.80 of prism base out.

Warning: Combining Oblique Cylinder and

Incorrect Interpupillary Distances

The total prism produced per eye is only 0.05 prism diopters. This amount is hardly measurable. The consequences of pushing the PD outward by 2 mm for a lens of this power are negligible.

Example 4-9

A lens blank is marked for the following prescription:

+5.00 –1.00 × 90

How much prism is induced by pushing the PD 2 mm outward?

Solution

When a lens contains a cylinder, the amount of prism induced depends upon total power in the meridian of decentration. This means that the orientation of the

Whenever a cylinder or spherocylinder lens having an oblique axis direction is decentered horizontally, a certain amount of vertical prism always is induced. The amount of vertical prism induced increases as the following occur:

1.The cylinder power increases

2.The cylinder axis approaches the 45or 135-degree position

In other words, moving an oblique spherocylinder horizontally would require a compensating vertical movement to counteract the vertical prismatic effect

8To find the power of a lens in a given meridian, it is helpful to use a power cross. For more on power crosses, see Chapter 16 in Brooks CW, Borish I: System for ophthalmic dispensing, ed 2, Boston, 1996, Butterworth-Heinemann.

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being induced. The amount of vertical compensation depends upon the cylinder power and axis direction. In short, when oblique cylinder is present, pushing the PD horizontally will result in some degree of vertical prism. This prism will not show up at the optical centers of the incorrectly placed lenses but will show up if the lens is checked at the location of the wearer’s PD.

AMOUNT OF REQUIRED ACCURACY

Accuracy required for the location of the OC may be specified either as millimeters of deviation from the ordered PD, or as the amount of prism induced by the incorrectly located OC, or both.

Low-powered lenses can have their OC off location by a relatively large amount before the prismatic effect becomes significant. However, as lens power increases, even small errors can cause a major prismatic effect. The American National Standards Institute (ANSI) has published agreed-upon Recommendations for Prescription Ophthalmic Lenses. These include prismatic effect and PD accuracy. (ANSI Z80.1 Prescription Ophthalmic Lens Recommendations may be found in Appendix B at the back of this book.)

Published standards are recommendations only; the practitioner may demand higher or accept lower standards. The supplying laboratory may remind an account of the generally accepted standards but should not attempt to hide behind them, as more precision in certain specified parameters is not an impossible task. If the practitioner and supplying laboratory cannot come to an acceptable agreement, then other sources of supply should be sought. Wearers of low refractive powers are often more sensitive to deviations from the absolutes required than wearers of high prescriptions, making higher standards well within reason in certain circumstances.

Standards for Interpupillary Distance and Unwanted Prism

The ANSI Recommendations for Prescription Ophthalmic Lenses (Z80.1) tries to balance unwanted prism amounts with what is realistically feasible. More than two thirds of a prism diopter of horizontal prism is

C H A P T E R 4 C E N T R AT I O N O F S I N G L E V I S I O N L E N S E S

undesirable. However, as in Prentice’s Rule, as the power of the lens increases, the amount of prism that is induced with a small movement of the lens center location also increases. The ANSI committee determined that for prescriptions of above plus or minus 2.75 D, it was realistically possible to require that the PD be within 2.5 mm of what was ordered. Unfortunately for high prescriptions this will induce more than two thirds of a prism diopter.

For unwanted vertical prism the ANSI Z80.1 standards state that no more than one-third prism diopter should be induced by any differences between left and right MRP heights. However, if the prescription is above plus or minus 3.375 D, the MRPs should be kept to within 1 mm of vertical difference for lens pairs or 1 mm of what was ordered for one lens only. This means that more than one-third prism diopter of vertical prism may result, even though the lens centers are vertically within 1 mm of each other. For these lenses, ANSI standards for vertical prism are still met.

ETHICAL FACTORS IN PRESCRIPTION ACCURACY

Because published prescription eyewear standards are recommendations only, in some instances a laboratory may be tempted to supply an account with prescription materials that fall outside the normally accepted standards when no one raises an objection. The laboratory supplying prescribed materials has an ethical responsibility to supply materials according to standard. Failure on the part of the account to check adequately for accuracy does not release the supplier from this responsibility. If anything, it places the ethical responsibility for the welfare of the wearer more directly on the laboratory.

Ophthalmic frames and lenses cannot be regarded as a commodity for which the consumer is the ultimate judge of acceptability. The laboratory that accepts materials that fall outside of normally accepted standards does not suffer from incorrect power or prism that was not prescribed. Wearers, who must trust in the judiciousness of those supplying their needs, are the ones who bear the consequences of dispenser and/or laboratory negligence.

Horizontal Decentration

1.What is the distance between centers (DBC) for a frame with the following dimensions?

A = 47

B = 39

DBL = 20

ED = 48

2.How much decentration, and in which direction, is required for each of the following?

(Note: 50 20 means the 50 mm eyesize and 20 mm bridge size is measured according to the boxing system.)

a.PD 66, 50 20

b.PD 60, 50 18

c.PD 59, 44 16

3.How much decentration per lens is required to correctly position the following lenses for edging?

R: +1.00 –1.00 × 70

L: +1.00 –1.00 × 100

A = 52

B = 49

DBL = 16

PD = 70

5.How much decentration per lens is required for a prescription with the following specifications?

A = 48

B = 38

ED = 48

DBL = 18

R monocular PD = 31.5

L monocular PD = 31.0

6.How much decentration per lens is required for the following prescription if it is to be used for reading glasses only?

R: +3.25 –0.50 × 90

L: +3.00 –0.25 × 90

A = 52

B = 47

ED = 57

DBL = 20

PD = 65/61

a.6 mm

b.4 mm

c.4.5 mm

d.3.5 mm

e.5.5 mm

7.How much decentration per lens is required if the person wearing these glasses will be wearing them for reading only?

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Vertical Decentration

8.The optical laboratory receives an order for a frame and lenses. The wearer’s PD is the same size as the (A) + (DBL) of the spectacle frame. The order specifies an MRP height of 23 mm for both lenses. If the frame has a vertical (B) dimension of 40 mm, how much vertical decentration is needed?

9.How much vertical and horizontal decentration per lens is required for the following single vision prescription?

R: –1.25 –0.75 × 15

L: –1.00 –1.00 × 162

Height of MRPs: 26 mm

PD = 66

A = 53

B = 48

ED = 57

DBL = 17

10.How much vertical and horizontal decentration of the major reference points (MRPs) of the lenses are required for the lenses that are to be placed in the following frame?

Minimum Blank Size

11.For the following frame and the wearer’s PD, which of the choices describes the minimum blank size required if 2 mm is allowed for lens chipping?

A = 49

B = 40

ED = 54

DBL = 20

PD = 61

a.57

b.66

c.59

d.64

e.None of the above

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12.What would the finished, single vision, minimum blank size be for a frame with an A dimension of 53 mm, a DBL of 18 mm, and an ED of 56 mm if the person who will be wearing these frames has a PD of 68 mm? (Note: Allow 2 mm for lens chipping.)

a.58 mm

b.61 mm

c.65 mm

d.69 mm

e.73 mm

13.An order for the same frame as in the question above is received, but it is for someone with a PD of 60. Which of the following lists the minimum blank size for this order if a finished, single vision lens blank is to be used? (Note: Allow 2 mm for lens chipping.)

a.58 mm

b.61 mm

c.65 mm

d.69 mm

e.73 mm

Z80 Standards

American National Standards Institute (ANSI) standards for prescription ophthalmic lenses have several different aspects. One of those is for the amount of unwanted horizontal prism that may be considered acceptable. Because lenses positioned for a wrong PD will induce horizontal prism, this is part of the standard as well.

14.A lens pair has a power of –5.25 D sphere for both right and left lenses. The wearer’s distance PD is 64 mm, but the glasses are made with a geometrical center distance of 62 mm. This will cause a total unwanted prismatic effect of 1.05

for both right and left eyes combined. Is this prescription acceptable according to ANSI standards in regards to PD/horizontal prism?

a.Yes

b.No

c.Not enough information given

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15.A lens pair has a power of –0.75 D sphere for both right and left lenses. The wearer’s distance PD is 60 mm, but the glasses are made with a distance of 66 mm between the optical centers. This will cause a total unwanted prismatic effect of 0.45

for both right and left eyes combined. Is this prescription acceptable according to ANSI standards in regard to PD/horizontal prism?

a.Yes

b.No

c.Not enough information given

16.A lens pair has a power of –7.25 D sphere for both right and left lenses. The right lens optical center is higher than the left lens optical center, but the vertical difference between the two optical center heights is just less than 1 mm. However, there is

0.5 of vertical prism that results. Is this prescription acceptable according to ANSI standards in regards to vertical prism/vertical MRP placement?

a.Yes

b.No

c.Not enough information given

Pushing the Interpupillary Distances

17.If the PD were “pushed” outward 1.5 mm per lens, what would be the total horizontal prismatic effect (right and left lenses combined) for each of the following prescriptions?

a.R: +2.00 D sphere

L:+2.00 D sphere

b.R: –4.00 –1.00 × 90

L:–4.00 –1.00 × 90

c.+5.25 –1.00 × 180

+6.25 –1.00 × 180

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General Questions

18.A right lens has a power of +5.00 –2.00 × 30. The frame chosen has dimensions of 52 20. The patient’s PD is 66. Assume that the uncut lens blank is facing up (convex side up). The OC is at the geometrical center of the blank. Which of the following represents where on the lens the center of the lens block would be located?

a.6 mm to the right of the geometrical center of the lens blank

b.6 mm to the left of the geometrical center of the lens blank

c.3 mm to the right of the geometrical center of the lens blank

d.3 mm to the left of the geometrical center of the lens blank

e.None of the above

19.Tony and Julie work at the same place. Both verify prescriptions. When a question exists about whether the prescription is acceptable, they both check the same one and compare notes. Tony checked the following prescription:

R: -2.00 – 3.50 × 45

L: -2.00 – 3.50 × 45

The PD was supposed to be 58 mm. He found the PD to be 66 mm and also found unwanted horizontal prism, but no vertical prism. He told Julie to check it as well. Julie found the same thing for PD and horizontal prism but said that she found unwanted vertical prism, too. Julie was right. Why?

a.Julie checked for vertical prism at the 66 mm location, and Tony checked at the 58 mm location. Because of the oblique cylinder, this made a difference.

b.Julie checked for vertical prism at the 58 mm location, and Tony checked at the 66 mm location. Because of the oblique cylinder, this made a difference.

c.Julie checked at both locations and averaged the difference.

d.It is impossible to tell why Julie found the unwanted vertical prism from the information given. Julie was obviously just better.

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20.Off-Center Optical Dispensers sent a prescription to Futile Vision Optical Laboratory. When the prescription was completed Randy from Futile Vision Optical verified their work and found that the PD was outside of standards and caused 1.5 of unwanted base in horizontal prism. A small amount of unwanted vertical prism also existed. Randy asked his manager what to do and mentioned that the prescription was for OffCenter Optical, to which his manager replied: “They never verify the work we send ’em. Send it

out like it is.” Sure enough, the job is sent out and never comes back. The wearer has difficulty in keeping her place when reading and has frequent headaches when doing extended near work. She attributes the headaches to stress. Which or who of the following is responsible?

a.Off-Center Optical and its employees and owners

b.Futile Vision Optical and its employees and owners

c.Both Off-Center Optical and Futile Vision and their owners and employees

d.The wearer, who did not follow up

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Challenge Questions

21.For the following prescription lens, how far from the center of the lens block on the uncut lens (i.e., the location of the geometrical center of the edged lens) will the optical center be located?

+4.00 D sphere 2 base out A = 50

B = 48

ED = 56

DBL = 18

PD = 60

22.A left lens has a power of –4.00 –1.00 × 180. The frame chosen has dimensions of 50 18. The patient’s PD is 60 mm. The prescription calls for 1.0 base down prism for the left eye. The OC is at the geometrical center of the lens blank. The uncut lens blank is convex side up (facing up). How far horizontally and vertically is the lens block center from the OC of the lens?